1. Field of the Invention
The present invention relates to switching arrays used for frequencies in a range between DC and microwave. More particularly, the present invention relates to an apparatus and a method for reducing crosstalk and dispersion in a crosspoint monolithic microwave integrated circuit (MMIC) switch array.
2. Background Information
Referring to FIG. 1, a crosspoint monolithic microwave integrated circuit (MMIC) switch array 100 includes thyristor switches 105, an upper dielectric layer 110 and a lower dielectric layer 115, with the lower dielectric layer 115 being positioned upon a substrate 120. Planar connections are made to the upper and lower levels of each thyristor 105. The substrate 120 is made from a semiconductor material, such as gallium arsenide (GaAs). The array is bounded above by an upper ground plane 125 and below by a lower ground plane 130. An example of such a crosspoint MMIC switch array is described in, for example, U.S. patent application Ser. No. 09/788,296 (hereinafter referred to as “T010”), filed on Feb. 16, 2001, entitled “Thyristor Switch for Microwave Signals”, and U.S. patent application Ser. No. 09/788,298 (hereinafter referred to as “T011”), filed on Feb. 16, 2001, entitled “Telecommunications Switch Array with Thyristor Addressing.”
The respective thicknesses h1 and h2 of the lower dielectric layer 115 and upper dielectric layer 110 can be, for example, 10 μm. The semiconductor substrate thickness hs can be approximately 150 μm and the pitch p can be 150 μm. The line width w for a 50 ohm line can be 20 μm. The thyristors 105 are shown as the mesa structures in the figure, and there is one thyristor 105 at each intersection of row transmission lines 135 and column transmission lines 140. For example, if a 64×64 crosspoint MMIC switch array is considered, such an array would have an overall dimension of 150 μm * 64=0.96 cm=approximately 1.0 cm, thus yielding a die size of approximately 1 cm×1 cm.
With a constant inter-electrode pitch p, crosstalk can be reduced by reducing the thicknesses h1 and h2 of the dielectric layers and reducing the semiconductor substrate thickness hs. This has the desired effect of reducing the line width w consistent with 50 ohm impedance. An additional benefit is increasing the gap between adjacent transmission lines 135 and 140, since the increase in the gap results in higher isolation and lower cross-talk between the adjacent transmission lines 135 and 140. This has the disadvantage, however, of increasing signal insertion loss, because of the increased ohmic losses of the traces, as well as leading to problems with fabrication.
In order to decrease hs to being less than 150 μm, it is necessary to etch the semiconductor material, such as GaAs, from the backside after the front-side processing is completed. However, in practice, it is difficult to reduce the thickness of the GaAs substrate sufficiently to where its thickness hs is small compared to that of the dielectric layer thicknesses h1 and h2 (e.g., 10 μm). Under laboratory conditions, GaAs can be thinned in small areas to approximately 50 μm, but in production, GaAs usually has a thickness which is greater than or equal to 150 μm to avoid breakage of brittle GaAs during subsequent handling.
One type of dielectric material that can be used is benzocyclobutene (BCB). BCB has a dielectric constant of 2.65. A low viscosity form of BCB, marketed as CYCLOTENE™ 4022 by Dow Chemical Company, has a maximum thickness of 5 μm and a high viscosity BCB, and CYCLOTENE™ 4026, also marketed by Dow Chemical Company, has a maximum thickness of 15 μm, both of which are the stress limits for these polymers. The result of the above constraints is that microwave transmission as described in the aforementioned U.S. Patent Applications, which occur throughout non-homogeneous media, such as GaAs and dielectric, yields considerable crosstalk and dispersion.
Therefore, there is a need for a design that reduces crosstalk while keeping the pitch relatively small, approximately on the order of 150 μm, so that 64×64 arrays are possible with a maximum dimension of 64*150 μm=1.0 cm.